This invention generally relates to electronic filters, and more particularly, to tunable microstrip line resonator filters.
The number of wireless communication systems has increased in the last decade, crowding the available radio frequency spectrum. Filter products used in radios have been required to provide improved performance with smaller size. Efforts have been made to develop new types of resonators, new coupling structures and new filter configurations. One of the techniques for reducing the number of resonators is to add cross couplings between non-adjacent resonators to provide transmission zeros. As a result of these transmission zeros, the filter selectivity is improved. However, in order to achieve these transmission zeros, certain coupling patterns have to be followed. This impedes the size reduction effort.
Electrically tunable microwave filters are highly desirable for communications applications. Magnetically and mechanically tunable filters are large and heavy. Electrically tunable filters use electrically tunable varactors in combination with the filter resonators. When the varactor capacitance is electrically tuned, the resonator resonant frequency is adjusted, which results in a change in the filter frequency response. Electrically tunable filters have the important advantages of small size, light weight, low power consumption, simple control circuits, and fast tuning capability. Traditional electronically tunable filters use semiconductor diode varactors. Compared with the semiconductor diode varactors, tunable dielectric varactors have the merits of lower loss, higher power-handling, higher IP3, and faster tuning speed. For most tunable filter applications, it is desirable to keep the filter configuration simple, otherwise it will be hard to tune the filter from one frequency to the other and still to maintain reasonable filter performance.
Tunable filters for wireless mobile and portable communication applications must be small in size and must have a relatively uncomplicated coupling structure. These design requirements mean that adding cross coupling to achieve transmission zeros, especially of the elliptic function type, is not a good option.
For miniaturization, a hairpin resonator structure has been widely used in microstrip line filters, especially for filters employing high temperature superconductor (HTS) materials. See for example, U.S. Pat. No. 3,745,489 by Cristal et al. for xe2x80x9cMicrowave And UHF Filters Using Discrete Hairpin Resonatorsxe2x80x9d. It has been noticed that such filters have a transmission zero near the low end of the operating frequency, which results in an improvement in the filter selectivity at the low frequency side, but a degradation in the filter selectivity at the high frequency side, even though, theoretical analysis shows that the transmission zero should be at the high frequency side. See, George L. Matthaei, Neal O. Fenzi, Roger J. Forse, and Stephan M. Rohlfing, xe2x80x9cHairpin-Comb Filters for HTS and Other Narrow-Band Applications,xe2x80x9d IEEE Trans. On MTT-45, August 1997, pp 1226-1231.
Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called xe2x80x9cferroelectricxe2x80x9d because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BSTO) or BSTO composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Materialxe2x80x9d; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-MgOxe2x80x9d; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTO-ZrO2xe2x80x9d; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled xe2x80x9cMultilayered Ferroelectric Composite Waveguidesxe2x80x9d; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled xe2x80x9cThin Film Ferroelectric Composites and Method of Makingxe2x80x9d; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled xe2x80x9cMethod of Making Thin Film Compositesxe2x80x9d; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled xe2x80x9cElectronically Graded Multilayer Ferroelectric Compositesxe2x80x9d; U.S. Pat. No. 5,635,433 to Sengupta, entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-ZnOxe2x80x9d; and U.S. Pat. No. 6,074,971 by Chiu et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materials with Enhanced Electronic Properties BSTO-Mg Based Compound-Rare Earth Oxidexe2x80x9d. These patents are hereby incorporated by reference. The materials shown in these patents, especially BSTO-MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
In addition, the following U.S. patent applications, assigned to the assignee of this application, disclose additional examples of tunable dielectric materials: U.S. application Ser. No. 09/594,837 filed Jun. 15, 2000, entitled xe2x80x9cElectronically Tunable Ceramic Materials Including Tunable Dielectric and Metal Silicate Phasesxe2x80x9d (International Publication No. WO 01/96258 A1); U.S. application Ser. No. 09/768,690 filed Jan. 24, 2001, entitled xe2x80x9cElectronically Tunable, Low-Loss Ceramic Materials Including a Tunable Dielectric Phase and Multiple Metal Oxide Phasesxe2x80x9d; U.S. application Ser. No. 09/882,605 filed Jun. 15, 2001, entitled xe2x80x9cElectronically Tunable Dielectric Composite Thick Films And Methods Of Making Samexe2x80x9d (International Publication No. WO 01/99224 A1); U.S. application Ser. No. 09/834,327 filed Apr. 13, 2001, entitled xe2x80x9cStrain-Relieved Tunable Dielectric Thin Filmsxe2x80x9d; and U.S. Provisional Application Serial No. 60/295,046 filed Jun. 1, 2001 entitled xe2x80x9cTunable Dielectric Compositions Including Low Loss Glass Fritsxe2x80x9d. These patent applications are incorporated herein by reference.
Examples of filters including tunable dielectric materials are shown in U.S. patent application Ser. No. 09/734,969 (International Publication No. WO 00/35042 A1), the disclosure of which is hereby incorporated by reference.
There is a need for tunable electronic filters that maintain structural simplicity, are relatively small, and provide transmission zeros.
An electronic filter constructed in accordance with this invention includes a first microstrip line hairpin resonator including first and second arms, a first varactor connected between a first end of the first arm and a first end of the second arm of the first microstrip line hairpin resonator, a first capacitor connected between a second end of the first arm and a second end of the second arm of the first microstrip line hairpin resonator, the first and second arms being coupled to provide a first transmission zero, an input coupled to the first microstrip line hairpin resonator, a second microstrip line hairpin resonator including third and fourth arms, a second varactor connected between a first end of the third arm and a first end of the fourth arm of the second microstrip line hairpin resonator, a second capacitor connected between a second end of the third arm and a second end of the fourth arm of the second microstrip line hairpin resonator, the third and fourth arms being coupled to provide a second transmission zero, and an output coupled to the second microstrip line hairpin resonator. The first and second arms and the third and fourth arms are substantially parallel to each other.
The capacitance of the varactors, and thus the frequency response of the filter, can be controlled by applying a control voltage to each of the first and second varactors. The first and second microstrip line hairpin resonators can be coupled to form a Chebyshev type of filter response. Each of the varactors can comprise a layer of tunable dielectric material, and first and second electrodes positioned adjacent to the layer of tunable dielectric material. The varactors can alternatively comprise a microelectromechanical capacitors or semiconductor diode varactors.
The invention also encompasses a resonator for an electronic filter comprising a first microstrip line including first and second arms, a first varactor connected between a first end of the first arm and a first end of the second arm of the first microstrip line, and a first capacitor connected between a second end of the first arm and a second end of the second arm of the first microstrip line, the first and second arms being coupled to provide a transmission zero.